1. Field of the Invention
The present invention relates to impact printers utilizing magnetically actuated hammers, and more particularly to impact printers in which hammer springs normally held magnetically in retracted positions against pole pieces are repeatedly released from the retracted positions to effect printing.
2. History of the Prior Art
Impact printers are known in which a plurality of magnetically actuated springs comprising a plurality of hammers repeatedly impact and reside against pole pieces during the operation thereof. An example of such a printer is described in U.S. Pat. No. 3,941,051 of Barrus et al., which patent is commonly assigned with the present application. The Barrus et al. patent describes a dot matrix line printer in which a plurality of hammer springs carried on a reciprocating shuttle assembly are magnetically actuated to provide the printing of dots. Each hammer spring which is of elongated configuration and of resilient magnetic material is mounted at a first end thereof so that an opposite second end thereof is free to move as the hammer spring flexes. The hammer spring is normally held in a retracted position against the tip of a pole piece by a permanent magnet which pulls the free second end of the hammer spring into contact with the tip of the pole piece. Momentary energization of a magnetic coil surrounding the pole piece causes the hammer spring to fly forward out of the retracted position, enabling a dot printing impact tip mounted at the free second end of the hammer spring to impact the print paper through an ink ribbon and thereby print a dot on the paper. Upon printing of the dot, the hammer spring rebounds into the retracted position against the pole piece tip where it is held in readiness for the next firing of the hammer spring.
Impact printers subject the hammers thereof to frequent and repeated firings during a printing operation. This is particularly true in the case of dot matrix printers such as of the type described in the previously referred to Barrus et al patent where each hammer spring in the hammer bank may be fired many times during each brief movement of the hammer bank across the width of the print paper. At the end of each such movement of the hammer bank, the hammer bank is reversed in direction and the process is repeated. Although the hammer springs and the pole pieces are made of hard, wear-resistant material such as high carbon steel, the repeated impacting of the pole pieces by the hammer springs eventually produces wear. Wear occurs at the portion of the hammer spring surface which repeatedly strikes and is held in contact with the pole piece. Wear also occurs at the interfacing tip of the pole piece.
As wear begins to occur at the pole piece tips and at the impacting surfaces of the hammer springs, both the physical and the magnetic characteristics of the hammers begin to change. Because of the extremely quick and repetitive nature of the hammer firings, the printer cannot tolerate much of a change in these characteristics. If the wear becomes excessive so as to alter the operating characteristics to a sufficient degree, the printer must be taken out of service until appropriate repairs are performed thereon. This may require replacement of the entire hammer bank with the worn hammer bank then having to be rebuilt at considerable expense.
In an effort to reduce wear at the interfaces between the pole pieces and the hammer springs, the tips of the pole pieces are frequently plated with a wear-resistant material such as chromium. This technique has had limited success in reducing the wear on the pole pieces and has done little if anything to alleviate the problem of hammer spring wear. In some cases, the presence of the chromium plating on the pole piece tips actually aggravates the problem of hammer spring wear.
In an effort to reduce hammer spring wear, attempts have been made to chrome plate the entire hammer spring, with unsatisfactory results. This appears to be due to the fact that chromium plating of the entire spring creates a bimetallic stress problem that adversely alters the mechanical characteristics of the spring.
Accordingly, it would be advantageous to provide a technique for greatly reducing wear to both the hammer springs and the pole pieces in an impact printer using hammer springs. It would furthermore be desirable to provide a protective plating for the hammer springs which has little or no effect on the operating characteristics of the springs. It would furthermore be desirable to provide a protective plating for the tips of the pole pieces which protects the pole pieces against wear without adversely affecting the wear of the hammer springs.